The present disclosure relates in general to the fields of optical microcavities, Brownian motors, electrostatic traps, particles detection, in particular the characterization of such particles, e.g., using Raman spectroscopy and related spectroscopy techniques.
Optical microcavities are known to confine light to a small volume. Devices using optical microcavities are today essential in many fields, ranging from optoelectronics to quantum information. Typical applications are long-distance data transmission over optical fibers, optical sensing and read/write laser beams in DVD/CD players. A variety of confining semiconductor microstructures have been developed and studied, involving various geometrical and resonant properties. A microcavity has smaller dimensions than a conventional optical cavity; it is often only a few micrometers thick and the individual layers that it comprises can even reach the nanometer range.
An optical cavity forms an optical resonator, which allows, in case of a Fabry-Perot geometry, a standing wave to form inside the central layer between the two mirrors. The thickness of the latter determines the cavity mode, which corresponds to the wavelength that can be transmitted and forms as a standing wave inside the resonator.
An optical resonator typically comprises a vertically layered stack of different materials and/or structures on a substrate that realizes two mirrors to confine the light in the vertical direction. Lateral confinement of the light is usually achieved by locally modulating the refractive index of the mirrors or the volume between them.
Various types of nanoparticle sensors have been proposed. For example, sensors are known, which involve plasmonic antennas (typically a pair of antenna elements), where the antennas define a hot spot volume (the active area between the antenna elements) in which particles must be brought to perform detection. Nanoparticle sensors are used for a range of applications, such as, e.g., the detection of pathogens, the surveillance of industrial processes, or environmental monitoring. Ideally, a nanoparticle sensor should be label-free, as labeling is specific and the label itself interferes with the assay and changes it properties.
For detection, a nanoparticle must be transported and trapped in the active area of the sensor. If the transport depends on diffusion only, the time required for a particle to diffuse to the detector becomes a limiting factor, especially at low concentrations. Detachment of the particles from the active area can further be an issue. If the transport is instead driven by a fluid flow, the fluid must be injected into the sensing area. This may require relatively high fluid pressures, which is difficult to handle. Moreover, particles may clog in the inlet channel, the outlet channel, and/or the sensing area.